1887

Abstract

In the yeast glucose or fructose represses the expression of a large number of genes. The phosphorylation of glucose or fructose is catalysed by hexokinase PI (Hxk1), hexokinase PII (Hxk2) and a specific glucokinase (Glk1). The authors have shown previously that either Hxk1 or Hxk2 is sufficient for a rapid, sugar-induced disappearance of catabolite-repressible mRNAs (short-term catabolite repression). Hxk2 is specifically required and sufficient for long-term glucose repression and either Hxk1 or Hxk2 is sufficient for long-term repression by fructose. Mutants lacking the gene, which encodes trehalose 6-phosphate synthase, can not grow on glucose or fructose. In this study, suppressor mutations of the growth defect of a double mutant on fructose were isolated and identified as novel alleles. All six alleles studied have single amino acid substitutions. The mutations affected glucose and fructose phosphorylation to a different extent, indicating that Hxk2 binds glucose and fructose via distinct mechanisms. The mutations conferred different effects on long- and short-term repression. Two of the mutants showed very similar defects in catabolite repression, despite large differences in residual sugar-phosphorylation activity. The data show that the long- and short-term phases of catabolite repression can be dissected using different hexokinase mutations. The lack of correlation between catalytic hexokinase activity, sugar phosphate accumulation and the establishment of catabolite repression suggests that the production of sugar phosphate is not the sole role of hexokinase in repression. Using the set of six mutants it was shown that there is a good correlation between the glucose-induced cAMP signal and hexokinase activity. There was no correlation between the cAMP signal and the short- or long-term repression of arguing against an involvement of cAMP in either stage of catabolite repression.

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/content/journal/micro/10.1099/13500872-145-3-703
1999-03-01
2019-09-16
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http://instance.metastore.ingenta.com/content/journal/micro/10.1099/13500872-145-3-703
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